Abstract
Abstract. The first wintertime in situ measurements of hydroxyl (OH), hydroperoxy (HO2) and organic peroxy (RO2) radicals (ROx=OH+HO2+RO2) in combination with observations of total reactivity of OH radicals, kOH in Beijing are presented. The field campaign “Beijing winter finE particle STudy – Oxidation, Nucleation and light Extinctions” (BEST-ONE) was conducted at the suburban site Huairou near Beijing from January to March 2016. It aimed to understand oxidative capacity during wintertime and to elucidate the secondary pollutants' formation mechanism in the North China Plain (NCP). OH radical concentrations at noontime ranged from 2.4×106cm-3 in severely polluted air (kOH∼27s-1) to 3.6×106cm-3 in relatively clean air (kOH∼5s-1). These values are nearly 2-fold larger than OH concentrations observed in previous winter campaigns in Birmingham, Tokyo, and New York City. During this campaign, the total primary production rate of ROx radicals was dominated by the photolysis of nitrous acid accounting for 46 % of the identified primary production pathways for ROx radicals. Other important radical sources were alkene ozonolysis (28 %) and photolysis of oxygenated organic compounds (24 %). A box model was used to simulate the OH, HO2 and RO2 concentrations based on the observations of their long-lived precursors. The model was capable of reproducing the observed diurnal variation of the OH and peroxy radicals during clean days with a factor of 1.5. However, it largely underestimated HO2 and RO2 concentrations by factors up to 5 during pollution episodes. The HO2 and RO2 observed-to-modeled ratios increased with increasing NO concentrations, indicating a deficit in our understanding of the gas-phase chemistry in the high NOx regime. The OH concentrations observed in the presence of large OH reactivities indicate that atmospheric trace gas oxidation by photochemical processes can be highly effective even during wintertime, thereby facilitating the vigorous formation of secondary pollutants.
Highlights
Air quality improvement has become a hot topic in China from citizens to government
The observed OH radical concentrations at noontime ranged from 2.4 × 106 cm−3 in severely polluted air to 3.6×106 cm−3 in relatively clean air
The BEST-ONE campaign was performed at Huairou at the northwest edge of Beijing in 2016 to elucidate pollution formation mechanisms in the North China Plain providing the first wintertime observations of OH, HO2, and RO2 radicals in this region
Summary
Air quality improvement has become a hot topic in China from citizens to government. a series of air pollution control provisions have been implemented by the Chinese government in the last 15 years to improve the air quality in China since early 2000. The additional HONO sources are effective at cold conditions and play an important role for the wintertime oxidation of pollutants in urban atmospheres. It confirms the need for in situ HONO measurements during field studies focusing on the investigation of atmospheric radical chemistry (Alicke et al, 2003; Su et al, 2011; Lu et al, 2012; Kim et al, 2014). The OH concentrations tend to depend strongly on j (NO2) This can be explained by HONO photolysis being the dominant primary source of OH radicals The OH observations do not show such a difference; j(O1D) and j (NO2) both exhibit the same good correlation coefficient
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